1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display apparatus comprising a two-terminal nonlinear device such as an MIM (Metal Insulator Metal) device.
2. Description of the Related Art
In recent years, liquid crystal display apparatuses have widely been used for displaying purposes in personal computers, word processors, terminal displays of office-automation equipment, television image display apparatuses and like applications by virtue of their advantageous characteristics such as low power consumption, thinness and lightness. Liquid crystal display apparatuses are expected to find wider use, particularly, as image displays of portable information terminal devices. An electronic book, which serves as a substitute of a conventional book formed by binding printed paper sheets, is one such information terminal device. According to the aimed specifications of a liquid crystal display apparatus for use in this device, the screen size is about 6 to about 7 inches, the definition is about 1024xc3x97768 dot XGA, and the operating temperature range is about xe2x88x9220 to 70xc2x0 C. An active matrix type liquid crystal display apparatus using an MIM drive has been disclosed in, for example, Japanese Unexamined Patent Publications JP-A 59-83190 (1984) and JP-A 9-54344 (1997).
FIGS. 9 and 10 illustrate part of the configuration of a conventional MIM-drive active matrix type liquid crystal display apparatus. FIG. 9 is a plan view of a partial configuration associated with one pixel, and FIG. 10 is a sectional view taken on line Xxe2x80x94X in FIG. 9. On an electrically insulating glass substrate 1 is formed a thin tantalum (Ta) film having a thickness of 3000 xc3x85 which will form a signal line 2 and a lower electrode 3 by sputtering or a like process. The thin tantalum film is patterned into a desired configuration to form the signal line 2 and the lower electrode 3 by photolithography. Subsequently, the surface of the lower electrode 3 is subjected to anodizing to form a 600 xc3x85-thick insulating film 4 comprising tantalum pentoxide (Ta2O5). On the entire surface of the substrate in this state is stacked a titanium (Ti) film, which will form an upper electrode 5, to a thickness of 4000 xc3x85 by sputtering or a like process, followed by patterning into a desired configuration by photolithography to form the upper electrode 5. In this way, there is formed a single MIM device 6 comprising the lower electrode 3, insulating film 4 and upper electrode 5.
Further, in the case where the liquid crystal display apparatus to be constructed is of the transmissive type, a transparent electrode film of ITO (Indium Tin Oxide) or a like material is stacked on the resulting structure and then patterned into a pixel electrode 7. Alternatively, in the case where the apparatus is of the reflective type, a reflective electrode film comprising aluminum (Al) or a like material instead of ITO or the like is stacked on the resulting structure and then patterned into a reflective pixel electrode, or, alternatively, a transparent electrode 7 of ITO or a like material is formed on the resulting structure, followed by affixing a reflective plate to the whole reverse side of the glass substrate 1. A plurality of such pixel electrodes are arrayed in a matrix shape, and signal lines 2 are routed to associated parts so that each pixel electrode 7 should be selectively driven through the associated MIM device 6. Similarly, pixel electrodes are formed on a counterpart glass substrate. The pair of substrates are mated with each other with their respective surfaces formed with respective pixel electrodes facing each other, and then a liquid crystal layer is placed between the pair of substrates to form the liquid crystal display apparatus.
FIGS. 11A and 11B illustrate an equivalent electric circuit configuration per pixel of an active matrix type liquid crystal display apparatus using an MIM drive and the voltage-current characteristic of an MIM device, respectively. In the equivalent circuit per pixel as shown in FIG. 11A, a parallel circuit including a resistor RMIM comprising the MIM device and a capacitor CMIM is serially connected to a parallel circuit including a resistor RLC comprising the liquid crystal layer and a capacitor CLC. When the liquid crystal layer is applied with a driving voltage V through the MIM device 6, a voltage VLC and a voltage VMIM are applied to the liquid crystal layer and the MIM device, respectively. The MIM device has the voltage-current characteristic as shown in FIG. 11B. As shown, the MIM device 6 exhibits a very large resistance and hence hardly allows a current to pass therethrough until the voltage VMIM at opposite ends of the MIM device 6 reaches a threshold voltage VTH. When the absolute value of the applied voltage VMIM exceeds the threshold voltage VTH, the MIM device 6 exhibits a decreasing resistance, while the voltage VLC applied to the liquid crystal layer increases to give rise to an electric field that changes the alignment of liquid crystals in the liquid crystal layer.
As described above, a liquid crystal display apparatus for use in an electronic book has a panel screen size of 5 to 7 inches and a definition as high as XGA, and operates within an operating temperature range of xe2x88x9220 to 70xc2x0 C. according to the specifications thereof. In implementing a liquid crystal display apparatus with a screen having such a size and such an XGA-grade definition, the wiring resistance of the routed electrodes and the charge addressing time raise a problem. With increasing wiring resistance, a signal applied is rounded to a greater extent and, hence, a higher driving voltage becomes required. As the location of an MIM device associated with each pixel becomes remoter from a terminal for driving the active matrix type display apparatus, the resistance of the wiring from such a terminal to the MIM device increases. Therefore, the lighting characteristic of the panel used as a liquid crystal display apparatus varies at different points of the panel which correspond to points at which differences in resistance arise. This results in a non-uniform display and like inconveniences. In the liquid crystal display apparatus described in Japanese Unexamined Patent Publication JP-A 59-83190 (1984), a pair of signal lines extending in opposite directions from a pair of terminal electrodes, respectively, are placed opposite to each other, and an MIM device is disposed between and connected to each of the signal lines and each pixel electrode. This arrangement described in this Gazette, however, aims to correct a pixel defect and, therefore, a driving signal is delivered to the pixel electrode from only one of the pair of signal lines via the associated MIM device in a normal state and, in case of the presence of a defective MIM device connected to the usually used signal line, the other signal line is used to deliver such a driving signal to the pixel electrode. This means that the Gazette does not disclose any arrangement to deliver driving signals to a pixel electrode from both of the pair of signal lines and, accordingly, a non-uniform display and like inconveniences cannot be prevented.
As a duty ratio increases with a higher definition, the charge addressing time per pixel is shortened. This results in degraded ON characteristic of MIM device 6 in particular. An active-matrix type panel in which one pixel electrode is provided with one MIM device 6 is usable within the operating temperature range of from about 0 to about 60xc2x0 C., or from about xe2x88x9220 to about 40xc2x0 C., and cannot be used within a wider temperature range above 60xc2x0 C. Japanese Unexamined Patent Publication JP-A 9-54344 (1997) discloses a liquid crystal display apparatus in which two MIM devices having different I-V characteristics are connected to one pixel electrode. This apparatus described in this Gazette, however, is configured to separately apply an on voltage and an off voltage for turning the liquid crystal on and off to a pixel electrode through respective MIM devices. This means that this Gazette does not teach any art of using the two MIM devices separately within different temperature ranges and, accordingly, the apparatus cannot be used within a wider temperature range.
Accordingly, an object of the invention is to provide a liquid crystal display apparatus which is capable of realizing a uniform display even with a high definition panel and which can be used within a wider temperature range.
The invention provides a liquid crystal display apparatus comprising a pair of substrates, a liquid crystal layer sandwiched between the pair of substrates, pixel electrodes arranged in a matrix shape on the substrates, and a plurality of two-terminal nonlinear devices provided for each of the pixel electrodes for selectively driving the pixel electrode, the two-terminal nonlinear devices being capable of separately driving the pixel electrode in different operating temperature ranges.
According to the invention, the two-terminal nonlinear devices selectively drive the pixel electrodes to realize a display of the liquid crystal display apparatus. The liquid crystal display apparatus has the plurality of two-terminal nonlinear devices for each of the pixel electrodes. Since the two-terminal nonlinear devices are different from each other in characteristics and are capable of separately driving according to different operating temperature ranges, a combination of these two-terminal nonlinear devices allows the liquid crystal display apparatus to be used within a wider temperature range.
In the invention it is preferable that the plurality of two-terminal nonlinear devices include a first two-terminal nonlinear device which allows a current equal to or higher than a first predetermined value to pass therethrough at a predetermined voltage, and a second two-terminal nonlinear device which allows a current equal to or lower than a second reference value which is smaller than the first reference value to pass therethrough at the predetermined voltage.
According to the invention, the plurality of two-terminal nonlinear devices provided for each of the pixel electrode include the first and second two-terminal nonlinear devices. The first two-terminal nonlinear device is formed to allow a current equal to or higher than the first reference value to pass therethrough at the predetermined voltage, while the second two-terminal nonlinear device is formed to allow a current equal to or lower than the second reference value which is smaller than the first reference value to pass therethrough at the predetermined voltage. By using the second two-terminal nonlinear device within a relatively high temperature range and the first two-terminal nonlinear device within a relatively low temperature range, the liquid crystal display apparatus, as a whole, can be used within a wider temperature range.
The invention also provides a liquid crystal display apparatus comprising a pair of substrates, a liquid crystal layer sandwiched between the pair of substrates, pixel electrodes arranged in a matrix shape on the substrates, two-terminal nonlinear devices for selectively driving each of the pixel electrodes, a signal line for delivering a driving signal to each of the pixel electrodes, and a terminal electrode provided at an end of the signal line, wherein the two-terminal nonlinear devices associated with each of the pixel electrodes have a resistance adjusted according to resistances of the signal line extending between the terminal electrode and the respective pixel electrode.
According to the invention, the resistance of the two-terminal nonlinear devices for each of the electrodes for selectively driving the pixel electrode is adjusted so that the difference between voltage drops at the respective pixel electrodes which occur at an application of a voltage should be made smaller, whereby influences due to the differences in resistance reflecting different signal line lengths can be absorbed, thus ensuring a display with less non-uniformity.
The invention yet also provides a liquid crystal display apparatus comprising a pair of substrates, a liquid crystal layer sandwiched between the pair of substrates, pixel electrodes arranged in a matrix shape on the substrates, first and second two-terminal nonlinear devices provided for each of the pixel electrodes for selectively driving the pixel electrodes, a first signal line for delivering a driving signal to each of the pixel electrodes via the first two-terminal nonlinear device, a second signal line for delivering a driving signal to each of the pixel electrodes via the second two-terminal nonlinear device, a first terminal electrode provided at an end of the first signal line, and a second terminal electrode provided at an end of the second signal line, wherein the first signal line and the second signal line are formed so that a total value of resistance of the first signal line extending from the first terminal electrode to the first two-terminal nonlinear device and resistance of the second signal line extending from the second terminal electrode to the second two-terminal nonlinear device is almost the same at the respective pixel electrodes, and each of the pixel electrode receives the driving signal from both the first and second signal lines.
According to the invention, the first signal line and the second signal line are formed so that a total value of resistance of the first signal line extending from the first terminal electrode to the first two-terminal nonlinear device and resistance of the second signal line extending from the second terminal electrode to the second two-terminal nonlinear device is almost the same at the respective pixel electrodes, and each of the pixel electrode receives the driving signal from both the first and second signal lines. This means that the total length of the first signal line and the second signal line from respective terminal electrodes to the associated pixel electrode is generally equalized throughout all the pixel electrodes to equalize influences due to voltage drops of driving signals delivered to the first and second signal lines throughout all the pixel electrodes, thereby realizing a display with less non-uniformity.
The invention further provides a liquid crystal display apparatus comprising a pair of substrates, a liquid crystal layer sandwiched between the pair of substrates, pixel electrodes arranged in a matrix shape on the substrates, two-terminal nonlinear devices for selectively driving the pixel electrodes, a terminal electrode to which a driving signal is delivered, a first signal line extending toward one side from the terminal electrode, an insulating film formed on the first signal line, and a second signal line formed on the insulating film, wherein the first and second signal lines are connected to each other at plural conductive portions, and the plural conductive portions each have a resistance adjusted according to resistances of the first and second signal lines between the terminal electrode and the respective pixel electrodes.
According to the invention, the second signal line formed on the insulating film, which in turn is formed on the first signal line, is connected to the first signal line at plural points, and the resistance between the terminal electrode and the two-terminal nonlinear device associated with each pixel electrode can be adjusted so as to be equalized throughout all the pixels. This arrangement is capable of lessening the difference in waveform between signals applied to respective pixel electrodes thereby realizing a display with less non-uniformity.